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Differences Between CO- and Calcium Triplet-Derived Velocity Dispersions in Spiral Galaxies: Evidence for Central Star Formation?

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Differences Between CO- and Calcium Triplet-Derived Velocity Dispersions in Spiral Galaxies: Evidence for Central Star Formation? MNRAS 446, 2823–2836 (2015) doi:10.1093/mnras/stu2256 Differences between CO- and calcium triplet-derived velocity dispersions in spiral galaxies: evidence for central star formation? Rogemar A. Riffel,1‹ Luis C. Ho,2,3 Rachel Mason,4 Alberto Rodr´ıguez-Ardila,5 Lucimara Martins,6 Rogerio´ Riffel,7 Ruben Diaz,8 Luis Colina,9 Almudena Alonso-Herrero,10 Helene Flohic,11 Omaira Gonzalez Martin,12,13,14 Paulina Lira,15 Richard McDermid,4,16 Cristina Ramos Almeida,12,13 Ricardo Schiavon,2,17 Karun Thanjavur,18 Daniel Ruschel-Dutra,7,12 Claudia Winge8 and Eric Perlman19 Affiliations are listed at the end of the paper Downloaded from Accepted 2014 October 24. Received 2014 October 24; in original form 2014 May 20 ABSTRACT http://mnras.oxfordjournals.org/ We examine the stellar velocity dispersions (σ ) of a sample of 48 galaxies, 35 of which are spirals, from the Palomar nearby galaxy survey. It is known that for ultra-luminous infrared galaxies (ULIRGs) and merger remnants, the σ derived from the near-infrared CO band heads is smaller than that measured from optical lines, while no discrepancy between these measurements is found for early-type galaxies. No such studies are available for spiral galaxies – the subject of this paper. We used cross-dispersed spectroscopic data obtained with the Gemini Near-Infrared Spectrograph, with spectral coverage from 0.85 to 2.5 µm, to obtain σ measurements from the 2.29 µm CO band heads (σ CO) and the 0.85 µm calcium triplet at Universidad de Chile on May 20, 2015 (σ CaT). For the spiral galaxies in the sample, we found that σ CO is smaller than σ CaT, with a mean fractional difference of 14.3 per cent. The best fit to the data is given by σ opt = (46.0 ± 18.1) + (0.85 ± 0.12)σ CO.This‘σ -discrepancy’ may be related to the presence of warm dust, as suggested by a slight correlation between the discrepancy and the infrared luminosity. This is consistent with studies that have found no σ -discrepancy in dust-poor early-type galaxies, and a much larger discrepancy in dusty merger remnants and ULIRGs. That σ CO is lower than σ opt may also indicate the presence of a dynamically cold stellar population component. This would agree with the spatial correspondence between low-σ CO and young/intermediate-age stellar populations that has been observed in spatially resolved spectroscopy of a handful of galaxies. Key words: galaxies: active – galaxies: kinematics and dynamics – galaxies: star formation – infrared: galaxies. relation a very useful alternative. Cosmological simulations sug- 1 INTRODUCTION gest that the central SMBH evolves together with the host galaxy The empirical relationship between the stellar velocity dispersion and plays a fundamental role in its evolution (Di Matteo, Springel (σ ) of the spheroidal component of galaxies and the mass of the & Hernquist 2005; Springel, Di Matteo & Hernquist 2005; Bower supermassive black hole (SMBH; M•) at their centre (e.g. Fer- et al. 2006; Nemmen et al. 2007), and this co-evolution may be the rarese & Merrit 2000; Gebhardt et al. 2000) has been extensively mechanism that leads to the M•–σ relation. The intrinsic scatter used to estimate M• in active and inactive galaxies. More direct in the relation (i.e. the range of M• found for a given σ ) therefore determinations of M•, through stellar kinematics within the black contains information about the processes of galaxy and black hole hole’s sphere of influence, or broad emission line measurements, evolution (e.g. Gultekin¨ et al. 2009). Besides the M•–σ relation, are only feasible for a limited number of objects, making the M•–σ stellar velocity dispersion measurements are also relevant for many other astrophysical applications, including the galaxy Fundamental Plane (e.g. Djorgovski & Davis 1987; Dressler et al. 1987;Falcon-´ E-mail: [email protected] Barroso, Peletier & Balcells 2002;Bernardietal.2003; Gebhardt C 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society 2824 R. A. Riffel et al. et al. 2003; Valluri, Merrit & Emsellem 2004), the metallicity–σ up to twice the value obtained for the NIR. For the remaining eight relation (e.g. Terlevich et al. 1981; Dressler 1984a; Bender, Burstein non-LIRG mergers, σ CaT is slightly larger than σ CO.Evenlarger & Faber 1993), the V/σ ratio that is an important criterion to discrepancies have been found for single nucleus ultra-luminous determine the dynamical state of early-type galaxies (e.g. Cox et al. infrared galaxies (ULIRGs), for which σ CaT can be three times larger 2006; Cappellari et al. 2007; Naab et al. 2013), etc. For all of these than σ CO (Rothberg et al. 2013). The discrepancies arise because reasons, understanding the factors affecting σ measurements is an σ CO, although conveniently insensitive to dust absorption, probes important issue. a luminous, young stellar disc, whereas σ CaT gives information Measurements of σ in galaxies have traditionally been done about an older stellar population that is more representative of the at wavelengths <1 µm, often using the ‘Mgb’ line at 0.52 µm, galaxy’s overall dynamical mass. CO-based σ measurements imply or the 0.85 µm calcium triplet absorption (e.g. Ho et al. 2009). that ULIRGs cannot be the progenitors of giant elliptical galaxies, Measurements based on stellar absorption lines in the infrared (IR), whereas CaT-based values are consistent with a range of final galaxy on the other hand, can also probe regions that are obscured by dust masses. at optical wavelengths, or different stellar populations from those The above studies have compared optical and CO-based σ revealed in the optical. For these reasons, several recent studies have measurements for early-type (E and S0) galaxies, and for galaxy compared σ values obtained from the fitting of stellar absorptions mergers and ULIRGs. Little information is available, though, for in the optical and in the near-IR (NIR) spectral regions, generally late-type (spiral) objects, so the range of morphological types in using the CO absorption band heads in the H and K bands (Silge & which the large differences between σ CO and σ CaT occur is not yet Downloaded from Gebhardt 2003; Rothberg & Fischer 2010;Vanderbekeetal.2011; known. We aim to rectify this situation by measuring σ CO and σ CaT Kang et al. 2013; Rothberg et al. 2013). in a sample of 48 nearby galaxies: 35 spirals, 7 lenticulars, and 6 Silge & Gebhardt (2003) presented stellar velocity dispersions ellipticals. We do this using slit spectroscopy covering simultane- measured from the 2.29 µm CO band head in a sample composed ously the CaT and 2.29 µm CO spectral regions. In Section 2, we of 18 lenticular (S0) and 7 elliptical galaxies. The resulting sigma describe the sample and the observational data, and in Section 3, values were compared with the literature values derived primarily we discuss the methods used to measure the stellar dispersion in http://mnras.oxfordjournals.org/ from fitting the Ca II lines near 4000 Å, the Mgb lines near 5175 the optical and in the NIR. The results are presented in Section 4, Å, and the 8500 Å Ca II triplet (hereafter CaT). The σ CO values while their implications are discussed in Section 5. The conclusions were found to be up to 30 per cent smaller than σ opt, with a median of the present paper are given in Section 6. difference of 11 per cent1. However, this difference was observed only in the lenticular galaxies; in the elliptical galaxies, the optical and IR measurements were consistent. The authors speculate that 2 OBSERVATIONS AND DATA REDUCTION the difference may be related to the presence and distribution of dust The sample of galaxies used in this work comprises 48 objects in the S0 objects: if the dust is mainly located in the stellar disc, selected from the Palomar spectroscopic survey of nearby galaxies optical σ measurements will be biased towards the less obscured, (Ho, Filippenko & Sargent 1995, 1997), covering a wide range of at Universidad de Chile on May 20, 2015 dynamically hotter bulge component <1 µm. luminosity and nuclear activity type. Some properties of the sample σ Vanderbeke et al. (2011) measured CO in a sample of 19 Fornax are shown in Table 1, while full details of the overall programme, cluster members, composed of roughly equal numbers of elliptical sample, observations, and data reduction are given in Mason et al. and lenticular galaxies. These measurements were compared with (submitted). σ opt from Kuntschner (2000) and found to be consistent, with σfrac = 2 σ −σ Briefly, the spectroscopic data were obtained in queue mode CO opt = 6.4 per cent. The lack of a discernible difference between σopt with the Gemini Near-Infrared Spectrograph (GNIRS) on the σ CO and σ opt in the lenticular galaxies does not agree with the Gemini North telescope. The cross-dispersed mode was used with findings of Silge & Gebhardt (2003). It is, however, consistent with the 32 l mm−1 grating, providing simultaneous spectral coverage a previous study of velocity dispersions in the Fornax cluster by from approximately 0.85 to 2.5 µm. We used the 0.3arcsec× Silva, Kuntschner & Lyubenova (2008). 7 arcsec slit, generally oriented along the mean parallactic angle Optical and IR velocity dispersions were also compared by Kang at the time of the observations to minimize differential refraction et al. (2013). This work used the CO absorption bands around 1.6 effects. The data were obtained between 2011 October and 2013 −1 µm, rather than those near 2.3 µm, to derive σ CO for 31 galaxies: May.
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